1. Field of the Invention
The present invention relates to a strobe unit having therein a light source and a reflecting mirror.
2. Description of Related Art
A known strobe unit has a reflecting mirror (shade) that is located behind a light source to reflect light from the light source, so that a composite bundle of rays includes reflected light and direct light from the light source is emitted from the strobe unit towards an object to be taken. The reflecting mirror is usually a semi-elliptical reflecting mirror which has an elliptical cross-sectional shape along a plane that is normal to an axis of a light emitting tube.
The inventors of the present invention have analyzed the orientation properties of a strobe unit having a semi-elliptical reflecting mirror and have found that the quantity of light at the center portion is smaller than that at the upper and lower portions thereof. On irregularity in the distribution of light, as a result of the orientation properties, has an adverse influence on the picture of an object and decreases the guide number of the strobe which is based on the quantity of light at the center portion.
There are two types of known strobe units. A first type has a Fresnel lens (condenser lens) positioned in front of a reflecting mirror and a second type has no Fresnel lens. The second type, having no Fresnel lens, has a high light utilization efficiency, since there is no vignetting by the Fresnel lens (light loss caused by the wedge portion of the Fresnel lens). Nevertheless, the orientation properties of the second type of strobe unit still exhibit an irregularity in the distribution of light. On the Other hand, it is difficult to miniaturize the first type of strobe unit because of the presence of the Fresnel lens.
The second type strobe unit, having no Fresnel lens, is usually incorporated in a camera having a lens of fixed focal length, while the first type strobe unit, having the Fresnel lens, is usually used as a zoom strobe device for a zoom lens camera. In the zoom strobe device, the movement of the light source and the reflecting mirror relative to the condenser lens in the optical axis direction causes the illumination angle to change. It has been found that the condenser lens in the strobe device decreases the illumination angle of direct light from the light source on the telephoto side, but there is little change in the illumination angle of light (indirect light) reflected by the reflecting mirror. Therefore, if optimum orientation properties are determined on the wide-angle side, there is a large quantity of light radiating beyond the angle of view on the telephoto side, resulting in a decreased guide number. This results in a phenomenon wherein, from the viewpoint of the orientation properties, the quantity of light at the center portion is less than that in the circumferential portion and the quantity of light emitted beyond the angle of view is increased.